Various means for removing toxic substances contained in the exhaust gases of internal combustion engines have been proposed. Among them is included apparatus that burns a lean air/fuel mixture. This is one of the very effective measures, achieved by taking notice of the fact that combustion of a lean air/fuel mixture, in which the ratio of air to fuel is high, reduces such toxic substances, especially nitrogen oxides, in the exhaust gases.
However, a lean air/fuel mixture usually gives rise to misfiring and lowers thermal efficiency of the engine cycle, because of the low ignitability and slow flame-propagation speed. To overcome this shortcoming, it has been proposed to provide an auxiliary combustion chamber in addition to the main combustion chamber, so that a lean air/fuel mixture in the main combustion chamber is ignited by the torch flame blown from the auxiliary combustion chamber. In a typical embodiment of this type, the auxiliary combustion chamber is equipped with an intake valve to admit a relatively rich air/fuel mixture, which is ignited by a spark plug, and then a lean air/fuel mixture in the main combustion chamber is burnt by the torch flame from the auxiliary combustion chamber. This type of arrangement is very effective but the construction of the auxiliary chamber is very complex because of the necessity of an auxiliary intake valve.
It has also been proposed that an auxiliary combustion chamber having no intake valve be positioned in the main combustion chamber. During the compression stroke, lean air/fuel mixture in the main combustion chamber is forced into the auxiliary combustion chamber, where it is ignited by a spark plug. Then, the flame produced is blown back into the main combustion chamber to burn the lean air/fuel mixture therein. This arrangement has the defects that the residual gases from the preceding strike remain in the auxiliary combustion chamber making the ignition on next strike difficult and the lean air/fuel mixture is, itself, difficult to fire.
In the internal combustion engine having an auxiliary combustion chamber of this type, additionally the temperature in the auxiliary combustion chamber becomes excessively high because the auxiliary combustion chamber is not only exposed to high temperature gases for a long time, since ignition and combustion starts therefrom, but also is subjected to the influence of heat radiated through the flame nozzle provided therein. As a consequence, such undesirable phenomena become liable to occur as premature ignition and a reduction in the durability of the auxiliary combustion chamber against high temperature. It is, therefore, necessary to provide the auxiliary combustion chamber with the capacity to withstand high temperature and to dissipate the heat accumulated therein.